Little Hawk, yes, the 2-party system does contribute. But at least until the election of Bill Clinton, I don't remember the partisan animosity being as great as it is now. The anger in the '60s and '70s was more generational and about the Viet Nam war than it was Republican v. Democratic Party.
At other times anger is directed at the incumbent party because of hard economic times (e.g., the gas rationing of the 1970s).
But I don't remember being as upset about my party losing a governorship or a seat in Congress or the Presidency until 2000 and 2004, when there were so many problems with our voting "system" and the true outcome of the election was questionable. Even after GWB was installed by the SCOTUS, I think most Americans were willing to accept him -- especially right after the 9-11 attacks.    But the more we have had high-profile figures like Dick Cheney (and now his daughter) and the media shock jocks I mentioned before spewing wild, hostile accusations about the opponent party, the more the animosity has grown.

For me, I still don't feel hatred towards Republicans or "conservatives" in general. But I have to admit that when I hear Sarah Palin or Michelle Bachman or Liz Cheney making outrageous accusations about Obama and the Democrats, my gut reaction is one of real anger and disgust.